1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to an active matrix liquid crystal display device.
2. Description of the Related Art
Active matrix liquid crystal display devices using active elements represented by thin-film transistors (TFTs) are becoming widely used as display terminals for OA equipment or the like because of their features of thin size and light weight as well as their high picture quality which compares with Braun tubes.
Display methods for such liquid crystal display devices are generally classified into the following two methods. In one of the methods, a liquid crystal is interposed between two substrates each having a transparent electrode, and a voltage is applied across transparent electrodes to drive the liquid crystal, whereby light which has entered the liquid crystal is modulated to display a picture. This method is currently adopted by almost all popular products. In the other method, two electrodes are arranged over one substrate and a liquid crystal is driven by an electric field generated between two electrodes nearly in parallel with substrate surfaces, whereby light which has entered the liquid crystal is modulated to display a picture. This method has the feature of remarkably wide viewing angles, and is mainly adopted by part of liquid crystal monitor products.
The feature of the latter method is disclosed in, for example, Japanese Patent Application Laid-Open No. 505247/1993, Japanese Patent Publication No. 21907/1988, Japanese Patent Laid-Open Nos. 160878/1994, 15650/1997, 225388/1995, 306417/1995, and U.S. Pat. Nos. 5,754,266, 2,701,698, and 5,910,271.
The former method which uses electrodes formed over a protective film is disclosed in Japanese Patent Laid-Open Nos. 165059/1993 and 323373/1993 and U.S. Pat. No. 5,334,859.
However, it has been discovered that if the latter type of liquid crystal display device having the above-described construction continuously displays a picture by supply of electricity, black spotted irregularities (hereinafter referred to as contamination spots) are generated in places in the display device. In addition, it has been discovered that such contamination spots easily occur in display devices using liquid crystals having cyano groups, as described in Japanese Patent Laid-Open Nos. 225388/1995 and 306417/1995.
It has been found out that the latter method has another problem. As described in Japanese Patent Laid-Open No. 306417/1995, the latter method can use a liquid crystal of low resistivity, but such a liquid crystal tends to easily absorb impurities, so that impurities in the liquid crystal flow into the display and form indefinite black irregularities, or are stored in an edge portion of a display pattern and form image retention (image persistence). The invention has been made in view of the above-described problems, and provides a liquid crystal display device which can realize a wide viewing angle, a high picture quality and a high reliability by preventing indefinite black irregularities (contamination spots) or the like which are problems peculiar to the mass-production of IPS liquid crystal display devices (IPS: In-Plane Switching mode).
The outline of a representative aspect of the invention disclosed herein will be described below in brief. There is provided an in-plane switching mode liquid crystal display device having a construction in which scanning signal lines, video signal lines, pixel electrodes and counter electrodes for displaying a picture are formed under a passivation film formed over one of a pair of substrates, while electrodes or lines each having either or both of a positive pole and a negative pole are formed over the passivation film, and the electrodes or lines for displaying a picture are connected to the electrodes or lines each having either or both of a positive pole and a negative pole, via through-holes.
According to this construction, it is possible to restrain spotted black irregularities (contamination spots) from occurring when protective-film defects exist in the electrodes and lines. Incidentally, in the invention, the term xe2x80x9celectrode and line on a negative sidexe2x80x9d means a scanning signal line. An electrode or line which is at a higher potential than the scanning signal line is termed xe2x80x9celectrode or line on a positive sidexe2x80x9d, and the term xe2x80x9celectrode or line on a positive sidexe2x80x9d means the electrodes or lines required to display a picture, such as video signal lines, pixel electrodes and counter electrodes.